<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>定时器</title>
</head>
<body>
<h1>0</h1>
<h2>0</h2>
<script>
  window.setInterval(f,1000)
  let count=0;
  function f() {
    count++;
    let h=document.querySelector("h1");
    h.innerText=count;

  }
  let x=0;
  //匿名方法开启定时器
  let timer=setInterval(function () {
    x++
    let h=document.querySelector("h2")
    h.innerText=x;
    if(x>=50){
        //停止定时器
      clearInterval(timer)
    }
  },100)


  //开启只执行一次的定时器\
  setTimeout(function () {
    x=50;

  },3000)
</script>

</body>
</html>